Light valves have been known for more than sixty years for modulation of light. Light valves have been proposed for use in numerous applications during that time including, e.g., alphanumeric displays, television displays, windows, sunroofs, sunvisors, mirrors, eyeglasses and the like to control the amount of light passing therethrough. Light valves of the type described herein are also known as “suspended particle devices” or “SPDs”.
As used herein, the term “light valve” is used to describe a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent. The walls have electrodes thereon, usually in the form of transparent electrically conductive coatings. The cell contains a light-modulating element (sometimes herein referred to as an “activatable material”), which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as “a liquid light valve suspension” or as “a light valve suspension”) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement. Hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state.
For many applications, it is preferable for the activatable material, i.e., the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging associated with a high column of liquid suspension, can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that, in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A “light valve film” as used herein refers to a film having droplets of a liquid suspension of particles distributed in the film or in part of the film.
U.S. Pat. No. 5,409,734 exemplifies a type of light valve film that is made by phase separation from a homogeneous solution. Light valve films made by cross-linking emulsions are also known. See U.S. Pat. Nos. 5,463,491 and 5,463,492, both of which are assigned to the assignee of the present invention.
To facilitate a better understanding of suspended particle devices produced in accordance with the present invention, a brief description is provided below of the features of SPD light valves formed in accordance with the prior art.
A variety of liquid light valve suspensions are well-known in the art and such suspensions are readily formulated according to techniques well known to one of ordinary skill therein. The term “liquid light valve suspension” as used herein means a “liquid suspending medium” in which a plurality of small particles are dispersed. The “liquid suspending medium” comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
Subject to certain limitations discussed hereinafter, the liquid suspending media useful in suspended particle devices prepared in accordance with the present invention may optionally include, i.e., in addition to one or more of the new liquid suspending media described herein, one or more “prior art” liquid suspending media previously known for use in such light valves, depending upon considerations such as the intended application and/or the operating parameters of the light valve. Such “prior art” media include, but are not limited to, those disclosed in U.S. Pat. Nos. 4,247,175; 4,407,565; 4,772,103; 5,409,734; 5,461,506 and 5,463,492. In general, one or both of the liquid suspending medium or the polymeric stabilizer (described below) dissolved in the medium is chosen so as to maintain the suspended particles in gravitational equilibrium.
The polymeric stabilizer, when employed, may be a single type of solid polymer that bonds to the surface of the particles but which also dissolves in the non-aqueous liquid(s) of the liquid suspending medium. Alternatively, there may be two or more solid polymeric stabilizers serving as a polymeric stabilizer system. For example, the particles can be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, together with one or more additional types of solid polymeric stabilizer that bond to or associate with the first type of solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Also, liquid polymeric stabilizers may be used to advantage, especially in SPD light valve films, as described for example in U.S. Pat. No. 5,463,492.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be either light-absorbing or light-reflecting in the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed particles of colloidal size. As used herein the term “colloidal” means that the particles generally have a largest dimension averaging about 1 micron or less. Preferably, most polyhalide or non-polyhalide types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which averages 0.3 micron or less and more preferably averages less than one-half of the wavelength of blue light, i.e., less than 2000 Angstroms, to keep light scatter extremely low.
As used herein, the term “decay time” means the time needed for an SPD film, after the power to the device is turned off, to change from 90% to 10% of its light transmission in the activated (ON) state. The term “rise time”, as that term is used herein, is the time needed, after the power is turned on, for the film to change from 10% to 90% of its light transmission in the activated state. It should be noted that 100% of the film's light transmission in its activated state need not be its maximum possible light transmission but can be whatever amount of light transmission is chosen by the user as convenient in view of the voltage, frequency and/or other parameters the user chooses to employ. Generally the rise time is two to three times faster than the decay time because the rise time is a function of the electric field produced by the applied voltage, whereas the decay time is usually determined by Brownian Movement.
As additionally used herein, a “fast” decay time is 250 milliseconds or less at 25° C., preferably 0.100 milliseconds or less at 25° C. Moreover, as also used herein, a “very fast” decay time is 50 milliseconds or less at 25° C. and preferably 20 milliseconds or less at 25° C. When added together, the rise time and decay time of the SPD film constitute the film's “response time” as the term is used herein. As a practical matter, reducing the decay time has the effect of also reducing both the rise time and overall response time of the film.
Depending on the intended application, it may be desirable for an SPD film to have a response time that is fast or not. In order to achieve the desired speed, it is desirable to have a wide choice of liquid suspending media so that liquids having many different viscosities are available because, in addition to other factors, the rise time, decay time and response time of a liquid suspension is each a function of the suspension's viscosity, which is generally a function of the viscosity of the liquid suspending medium or media of the suspension. That is, if other factors are constant, the rise time, decay time and response time of a liquid suspension will all increase if the viscosity of its liquid suspending medium is increased, and conversely all of these times will decrease if the viscosity of the suspending medium is reduced.
A wide variety of liquids, both non-polymeric and polymeric, have been suggested in the prior art for use as the liquid suspending medium of an SPD light valve, or at least as a component thereof. The non-polymeric liquids include, without limitation, a variety of halogenated liquids, lower esters and plasticizers. Polymeric liquids previously disclosed include, without limitation, the polymeric liquid copolymers mentioned in U.S. Pat. No. 5,463,492 and polydimethylsiloxane.
In order for a liquid to be useful as a liquid suspending medium (or a component thereof) in a droplet of an SPD film, the liquid must (a) be substantially or completely immiscible with the matrix polymer used in forming the film, even at elevated temperature, e.g., 90° C.; (b) have a reasonably high boiling point (greater than 100° C., preferably greater than 150° C., and more preferably greater than 200° C.) at atmospheric pressure; be liquid at 25° C. and preferably at 0° C. and more preferably at −20° C. and below. Moreover, the liquid suspending medium as a whole should have an index of refraction that differs from the index of refraction of the matrix polymer by 0.002 units or less, preferably by 0.001 units or less at 25° C. In addition, the liquid should have an electrical resistivity of at least 0.8×106 ohms per square and preferably greater than 5×106 ohms per square. Some liquid suspending media have electrical resistivities of 1012 ohms per square or more.
If an SPD film is of the type made by phase separation from a homogeneous solution (as described for example in U.S. Pat. No. 5,409,734), then the liquid suspending medium of the droplet should be immiscible with the precipitated (phase separated) matrix polymer. If the SPD film is of the type where the film is formed by cross-linking (i.e., radiation curing or heat curing) the matrix, then the liquid suspending medium needs to be immiscible with both the uncured and the cured film.
The present invention is directed to improved films for use in suspended particle devices such as light valves. In particular, the invention is directed to new suspending media for use in forming such films, as well as to SPD light valves incorporating the improved films of the invention.